21st ICFA Beam Dynamics Workshop on
Laser-Beam Interaction
12.Jun,2001

Positron Production
from a Tungsten Single Crystal
at the KEK 8-GeV Electron Linac
Presenter: K.Sasahara
Graduate School of Science,Department of Physics,
Tokyo Metropolitan University

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

Research Organization
K.Sasahara, R.Hamatsu, S.AnamiA A.EnomotoA, K.FurukawaA，K.KakiharaA，T.KamitaniA，Y.OgawaA，S. OhsawaA，T.OogoeA，T.SuwadaA，H.OkunoB， K.UmemoriC，T.FujitaC, K.YoshidaC
V.AbabiyD，A.P.PotylitsinD，I.E.VnukovD
Graduate School of Sicence, Department of Physics,
Tokyo Metropolitan University, Tokyo, Japan
Accelerator Laboratory, KEKA, Tokyo, Japan
Institute of Particle and Nuclear Studies, KEKB, Tokyo, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima UniversityC, Japan
Nuclear Physics Institute, Tomsk Polytechnic UniversityD, Tomsk, Russia

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

1. Purpose of the Experiment
Study of the possibility of using a tungsten(W) single crystal as a target for positronproduction in a linear accelerator
Increase of the positron production efficiency
Optimum target thickness, etc…

⇒ Application to KEKB and future Linear Colliders

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

2. Principle
Coherent Bremsstrahlung and Channeling Radiation(Fig.1.)

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

3. The Method Using a Single Crystal(R.Chehab et al.)(Fig.2.)(a) Radiation and pair creation in one crystal(b) Radiation in the front crystal and pair creationin the amorphous converter21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

4. Experiment at the KEK 8-GeV Linac(1) Experimental Method and Apparatus(Fig.3.)

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o nRear View

Front View
(Fig.4.)

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L a s e r - B e a m I n t e r a c t i o n(2) Condition¾ Incident Electron Beam
Energy:8 GeVIntensity:
0.2 nC/bunch (~109e-)
Repetition:
2 Hz
Bunch Width:
10 ps
Beam Size at Target: 1.5 mmφ
Beam Divergence:
15 μrad (Vertical)
72 μrad (Horizontal)¾ Target for Positron Production
Tungsten Single Crystal：<111>axis
2.2 mm (Mosicity:1.5mrad ) & 9 mm (Mosicity:0.5mrad)
Tungsten Amorphous: 0~18 mm

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L a s e r - B e a m I n t e r a c t i o n¾ Positron SpectrometerPe+[MeV/c]10Acceptance[MeV/c·Steradian]
2.47×10-4

15

3.80×10-4

20

4.81×10-4¾ Positron Detector
Acrylic Cherenkov Counter，Lead-Glass Calorimeter21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

5. Experimental Results(1) Rocking Curve for Pe+=20MeV/c(Fig.5.)

Cf. Lindhart Angle: ~0.4 mrad
Multiple Scattering Angle: ~1.3 mrad@2.2mm, ~2.8 mrad@9mm

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n(2) EnhancementDef. The ratio between yields from the oriented (On-Axis)
and disoriented (Off-Axis) W crystal axis <111>.

(Fig.6.)

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L a s e r - B e a m I n t e r a c t i o n(3) Target Thickness Dependence
for Pe+=20MeV/c(Fig.7.)

(4) Comparison with the past experiment(Table.1.)

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

6. Summary
1. The enhancement of the positron yields for Pe+=20MeV/c,Thin W crystal (2.2mm) -☞ 5.1 times
Thick W crystal (9mm) -☞ 1.7 times
has been observed in the 8GeV electron beam. (see Fig.5)
2. As the momentum becomes low, the enhancement isgetting larger. (see Fig.5,6)⇒ Advantage for the capture efficiency of a Linac positron
generator (∵2)

21 s t ICFA Beam Dynamics Workshop on
L a s e r - B e a m I n t e r a c t i o n

3. The enhancement decreases as the target thickness
increases. (see Fig.6, Table.1)
4. The enhancement increases as the incident electronenergy increases. (see Table.1)
5. 9 mm-thick W crystal is comparable to 14 mm-thick W
amorphous, which is the optimum thickness for thepositron production at the KEKB injector Linac. (see Fig.7)⇒ When the energy of an incident electron beam
becomes high , it is expected that the positron yield
increases. (∵1,4,5)